Kan "Ja" i et felt i formular A generere en tekst i underformular B?

Answer to your question "Kan jeg ved at sætte flueben (=Ja) i et Ja/Nej-felt - felt A - i formular A få genereret en bestemt stump tekst i et tekstfelt - Felt B - i formular B?"

would be yes, but theres a couple of things you need to think about.

When you change the value of the yes/no field it hasnt actually changed the value in the table until the changes are commited to the table. IE after the forms AfterUpdate event is fired.

If its a one to many, is it all records in the sub form which need to be updated, or a specific?
And what do you do if you later change the value from Ja to Nej?

Til det, jeg skal bruge det til, er det ikke et problem, at ændringen af ja/nej feltet først tilføjes tabellen efter at posten bliver opdateret.

Når jeg sætter fluebenet til Ja i en given post, er det kun i tekstfeltet i denne specifikke post, der skal have tekstsumpen tilføjet.

So the sub form (B) is only showing one of the many records in table B?

Sounds strange that if you update a field on the main form (A) it only has to have an effect on one recod in B.
What if you now move to the next record in the sub form, the Ja/Nej field on the main form is still Ja!
Can you explain the logic in that?

Anyway, you should be able to change the value of a field on the sub form something like this

Me!FormB.Form!ControlName = "Some text"

Åh nej. Jeg har set og skrevet forkert. Både Ja/Nej felt og tekstfelt befinder sig i samme formular. Sorry!
At formularen er en underformular til en anden formular er vel sagen helt uvedkommende.

Det at felterne er i samme formular forenkler forhåbentligt problematikken.
Jeg beklager at have spildt din tid grundet fejlen!

No problem :-)

In the Ja/Nej fields afterupdate even you need some vba code similar to this

If Me.MyYesNo Then

    Me.someTextField =  "Yes..."
Else
    Me.someTextField =  "No..."

End If

MyYesNo  needs to be changed to the name of Ja/Nej field
and someTextField  to name of text field

Bingo! Det virker som det skal.

Tak for hjælpen og for din overbærenhed :-)

super, og selv tak